Function collate_refs 

pub fn collate_refs(
    meshes: &[InstanceMeshRef<'_>],
    min_group: usize,
    rtc: [f64; 3],
) -> Collated

Group instanceable meshes by representation identity into templates + per-instance transforms. min_group is the smallest occurrence count worth instancing (groups below it are emitted flat); use 2 to instance any repeat.

rtc is the model RTC offset (InstanceMeta.transform is documented pre-RTC at georeferenced magnitude, while each mesh’s baked origin is post-RTC and small). The per-occurrence relative transform is computed in the post-RTC frame: on raw absolute placements rel = m_k · m_ref⁻¹ lets a rotated occurrence’s translation reach T_k − R_rel·T_ref ≈ 2× rtc (the two ~1e6 m terms add instead of cancel), which then places the occurrence at twice the georeference and collapses f32 GLB exports. Reducing both transforms by rtc first keeps rel.translation at building scale and consistent with the small template origin.

InstanceMeta.transform (hence rel) is native-frame (IFC Z-up); positions must share that frame — see collate_refs_in_basis otherwise.